Coke is the solid carbonaceous material that can be made in many ways, including the destructive distillation of low-ash, low-sulfur bituminous coal, from cracking petroleum or from pyrolysis of biomass. Coke has many commercial uses and applications, the largest of which is as a fuel. However, it can also be undesired, fouling lines and clogging systems.
The coking behavior of a refinery hydrocarbon stream can be difficult to predict. First of all, coking is a subtle process, occurring very slowly after large amounts of hydrocarbon are exposed to heated metal. It is difficult to achieve a laboratory method to simply and accurately predict the coking reaction that will occur. Secondly, there are few good ways to rank the effectiveness of coking inhibitors. The reactions are slow, chemical amounts are low, and the affected metal surface areas are small. For example, Conradson carbon residue (CCR) measurements can indicate coking potential, but is time-consuming. Other instrumental monitoring methods for the coke lay-down reaction can also be used, but these tend to be expensive. To overcome the inherent limitations of prior techniques, either the reaction times or the sample sizes have to be large, thereby compensating for the relatively small reaction zone.
Although difficult, predicting coking behavior and inhibition are growing more important. The lighter grades of crude oil produce the best yields of fuel products, but as the world's reserves of light and medium oil are depleted, oil refineries increasingly need to process heavy oil and bitumen, using more complex and expensive methods to produce the products required. Because heavier crude oils have too much carbon and not enough hydrogen, coking is a more serious issue for heavy crude oils than it is for light crude oils. Refining generally involves removing carbon from or adding hydrogen to the molecules, and using fluid catalytic cracking to convert longer, more complex molecules to shorter, simpler ones for fuels.
U.S. Pat. No. 6,294,387 describes a method for determining corrosiveness of naphthenic acid in a fluid, including providing a fluid containing naphthenic acid; providing iron powder having a surface area of at least about 0.01 m2/g; contacting the fluid and the powder for a period of time so as to provide a portion of the iron dissolved in the fluid; and measuring iron concentration of the fluid containing the dissolved iron, so to measure corrosion potential for the naphthenic acid over time. However, to our knowledge iron powder has never been used to assess coking potential before.
What is needed in the art is a one pot, simple, inexpensive and reliable method of predicting coking behavior or assessing coke inhibitors.